There are significantly improved system peak rates required in a Long Term Evolution-Advanced (LTE-A) system up to 1 Gbps in the downlink and 500 Mbps in the uplink as compared with the Long Term Evolution (LTE) system; and also the LTE-A system is required to be well compatible with the LTE system, and there is a bandwidth up to 20 MHz in the LTE system. Carrier Aggregation (CA) has been introduced to the LTE-A system so as to accommodate the required improvement of the peak rates, compatibility with the LTE system, and full use of frequency resources.
For a User Equipment (UE) with a support of CA, resources of one or more cells can be aggregated concurrently for the UE, and the UE can transmit data concurrently in these cells. The number of cells aggregated for the UE is configured by a base station according to a service demand of the UE. In the Release 10 (R10)/Release11 (R11), each of the cells aggregated for the UE can include a pair of uplink/downlink carriers or only a downlink carrier, that is, the number of downlink carriers shall be more than or equal to the number of uplink carriers, but the number of uplink carriers shall not be more than the number of downlink carriers.
Cells with their carriers being aggregated are further categorized in the LTE-A system so that the cells configured for the UE can be categorized into a Primary Cell (PCell) and a Secondary Cell (SCell) dependent upon different functions of the respective cells aggregated for the UE. In the scenario where the carriers are aggregated, only one of the cells for each UE is defined as a Primary Cell, the Primary Cell is selected by the base station and configured to the UE in Radio Resource Control (RRC) signaling, and the Primary Cell is primarily responsible for carrying transmission of all the uplink control information of the carriers for the UE; and the cells aggregated for the UE other than the Primary Cell will be referred to as Secondary Cells, the Secondary Cells are configured by the base station and primarily responsible for transmitting traffic data of the UE.
Such a scenario where carriers are aggregated has been defined in the LTE R10 that there is a heterogeneous network including a macro eNB and local nodes, where the macro eNB provides underlying coverage, and the local nodes provide hotspot coverage. The local nodes include Remote Radio Heads (RRHs), repeaters, local eNBs, etc. In the scenario where the carriers are aggregated in the heterogeneous network, downlink data of the UE can be transmitted to the UE by only one transmitting node or can be transmitted to the UE concurrently by two transmitting nodes; and similarly uplink data of the UE can be transmitted to only one transmitting node or can be transmitted concurrently to two transmitting nodes. However from the perspective of a radio frequency design, it is relatively easy to receive data transmitted from two transmitting nodes concurrently in the downlink, but it may be relatively difficult to transmit data concurrently to two transmitting nodes in the uplink. At present there is generally only one transmitter for the UE in the uplink, that is, there is a support of only transmission of uplink data to a single transmitting node at a time.
For a UE with a support of only uplink transmission to a single node at a time, uplink transmission of the UE is generally active only with the PCell in the existing uplink transmission mechanism. In the heterogeneous network, the macro eNB provides a large coverage area, so a cell served by the macro eNB is typically selected as the PCell. The existing uplink transmission mechanism suffers from the drawbacks of an increased burden on the macro eNB, increased power consumption of the UE, increased uplink interference and a degraded throughout of the system.
In summary, the existing mechanism of uplink transmission by the UE may come with an increased burden on the macro eNB, increased uplink interference between UEs, and degraded power saving of the UE.